Which of the following statements best describes the structure of RNA molecules in general?

a. RNA molecules form a double helix modeled after the DNA from which they are encoded.
b. RNA molecules are single stranded but they commonly form localized secondary structures by forming covalent bonds between regions of the molecule.
c. RNA molecules are single stranded and they do not commonly form localized secondary structures.
d. RNA molecules are single stranded but they commonly form localized secondary structures by base pairing between regions of the molecule.